1. Field of the Invention
The present invention relates to antennas having three dimensional lobe patterns formed so as to attenuate signals received from objects which are within about 10 to 15 degrees of horizontal to substantially prevent GPS signals which are reflected off of ground objects from being received.
2. Description of the Prior Art
Global positioning systems (GPS) utilize signals from orbiting satellites to determine aircraft position. Differential global positioning systems (DGPS) are used around airports for guiding aircraft into landing patterns and landing. DGPS often utilize a ground based antenna to receive signals from the satellites and, knowing the ground based position exactly, provide corrections signals to the landing aircraft so that it may determine its position far more accurately. The antennas used in such ground based systems have a difficulty in distinguishing signals received directly from the satellites from those received by reflection from nearby objects such as the ground, buildings and trees (sometimes referred to as the multi-path problem). Accordingly, a number of efforts have been made to find ways of blocking or attenuating such reflected signals. A single patch antenna surrounded by truncated ground planes or a shaped blocking ring around the antenna has been used to alter the response to signals which are near the horizon level but to date, sufficient rejection of low angle signals while maintaining high gain for desired signals has not been achieved. Other attempts have been to use linear arrays of dipoles in combination with patch antennas and by altering the response to low level directions provide a lobe pattern which excludes multi-path signals. Such attempts have yet to be shown to be satisfactory in cost/performance for DGPS systems.
The present invention uses a flat plate with a plurality of spaced patch antennas with controlled phases and amplitudes during signal summation and so positioned that a melon-shaped lobe pattern is obtained with high attenuation at just above the horizon and high gain above the attenuation region.